挖掘机结构培训教材

nullnull 主讲： 刘军士 speaker:Liu junshi液压挖掘机液压挖掘机 液压挖掘机 培训 焊锡培训资料ppt免费下载焊接培训教程 ppt 下载特设培训下载班长管理培训下载培训时间表下载 教材 Hydraulic excavators training materials null　　　　　一 概论 general Introduction 1 定义：挖掘机是一种主要用于开挖土石方的施工机械。 Definition : An excavator is used in the excavation of earth and construction machinery 2 工作过程：它是用铲斗上的斗齿切削土壤并装入斗内，装满土后提升铲斗并回转到卸土地点卸土，然后再使转台回转、铲斗下降到挖掘面，进行下一次挖掘。 Working Scoop :It uses the tooth of the bucket to cut soil and packed inside the bucket. Fill the bucket with soil and upgrade back to the point of dumping, and then rotate the platform, drop the bucket to the tap surface,then the next excavation. 3 用途：挖掘机在建筑、筑路、水利、电力、采矿、石油、天然气管道铺设和军事工程中被广泛地使用。 Excavators uses : in building roads, water, power, mining, oil and gas pipeline laying project has been the widespread use of the military. null4 主要作业对象：挖掘机主要用于筑路工程中的路基开挖，建筑工程中开挖基础，水利工程中开挖沟渠、运河和疏浚河道，在采石场、露天开采等工程中剥离和矿石的挖掘等。据统计，工程施工中约60％的土石方量是靠挖掘机完成的。 Main operating target for major highway projects : the excavator used in subgrade excavation, construction excavation foundation Excavation of water ditches, canals and river dredging, quarries,opencast mining projects such as stripping and ore mining.According to statistics, about 60% of earthwork construction is completed by the excavator 5 其它用途：挖掘机更换工作装置后还可进行浇筑、起重、安装、打桩、夯土和拔桩等作业。 Other uses : replacement of the devices also catch excavators, cranes, the installation of the pile, Ramming and the Diocesan piling operations.null6 工作条件 Working conditions 6.1 液压挖掘机正常工作环境温度为-20℃～55℃。 Hydraulic excavators normal working temperature: -20 ° C ~ 55 ° C 6.2 液压挖掘机应能在Ⅲ级土壤中正常作业，对冻土层和岩石，需松裂和爆破后方可施工，且须使用岩石型铲斗进行作业；岩石型铲斗斗齿须满足JG/T 90-1999 的要求. Ⅲ soil hydraulic excavator should be able to work normally, and the rock layer of permafrost, and the need to split the song blasting before construction.Rock requires the use of bucket for operations; Rock Bucket Tooth JG/T 90-1999 must meet the requirements null6.3 轮胎式挖掘机在岩石工况作业时，对轮胎进行必要的保护。 Tire excavator is operated in rock condition, the tires should have necessary protection. 6.4 海拔3000 m能正常工作。 altitude of 3,000 m work normally 6.5 能在沙漠、盐碱地区工作。 Work normally in the desert, saline district 6.6 适于各种湿度地区工作，最大相对湿度要满足100％。In humidity areas,they are also suitable for all kinds of work, to meet 100% relative humidity. nullnull 二 挖掘机的发展史 The history of the excavators’developmentnull2.1 国外挖掘机的发展史 1）1837年第一台挖掘机在美国诞生----以蒸汽作动力的动力铲 2）1900年第一台以柴油机作动力的挖掘机问世 3）1954年第一台全液压挖掘机在德国出现 4）1961年日本从欧美引进了液压挖掘机技术 5）1985年第一台全电脑控制的液压挖掘机在日本出现 Excavator foreign history of the development: 1837 the first excavator in the United States was born-to steam-driven power shovel in 1900 the first diesel engine to power for the excavator made In 1954 first hydraulic excavator in German In 1961 Japan introduced a hydraulic excavator technical from Europe and the United States In 1985 the first of all computer-controlled hydraulic excavators was come into being in Japan null2.2 国内挖掘机的发展史 1）1954年生产出第一台机械式挖掘机 2）1960年开始研制液压挖掘机 3）1970年以后测绘国外样机仿制全液压挖掘机取得成功 4）1982年起挖掘机行业第一次全面引进德国公司的液压挖掘机技术及配套件技术The history of the development of domestic excavator in 1954 produced the first mechanical digger in 1960 began developing hydraulic excavator in 1970 after mapping foreign prototype hydraulic excavator successful imitation since 1982 Excavator the first German company to import the technology and accessories hydraulic excavator technology null5）1990年机械工业部组织引进日本小松液压挖掘机技术 6）1990年玉柴研制成功了机重1.3吨的微型挖掘机 7）1993年国外二手机开始在中国内地大量销售使用 8）1995年-1998年国外10家著名公司开始在中国建合资与独资企业 9）1998年上海建筑机械厂破产被上海彭浦机械厂兼并 5) 1990 Machinery Industry Ministry and Japan Komatsu hydraulic excavator technology 6) 1990 Yuchai success Machinery 1.3 tons of mini excavators 7) 1993 foreign two cell phones in mainland China selling large quantities of used 8) 1995 -1998, 10 foreign well-known companies in China, joint ventures and wholly-owned enterprises 9) in 1998 Shanghai Construction Machinery Works bankruptcy Shanghai Effectiveness Evaluation on the Pengpu Machinery Plant merger null三 挖掘机的分类Excavators classificationnull3.1 按机重分类可分成： Classification by heavy machine can be divided into : 微型（小型） micro (small) G ≤6t 中小型 mid and micro 6 t＜G ≤16t 中型 Medium 16 t＜G ≤40t 大型 Large 40 t＜G ≤100t 特大型 vast Large G ＞100tnull国内外普遍使用原则是： Home and abroad commonly used are 40t以上的大型机以正铲为主，单用途：挖掘the excavators more than 40t mainly by front shovel,single purpose: excavating 6t以下的小（微）型机以履带（全部）反铲为 主，多用途 the excavators below 6t mainly crawler and backhoe,multi-purposes. 10t-22 t之间的挖掘机轮式与履式混用，以履式占绝大多数，挖掘为主兼 顾多用途. 10-22t’s excavators have wheeled and crawler ones,mainly are crawlers, which have multi-purposes. 22 t以上的挖掘机全部为履带式挖掘机 all of the above 22t excavators are crawler excavatorsnull 3.2 按传动方式分类： by means of driving can be classified 液压传动挖掘机和机械传动挖掘机 hydraulic and mechanical transmission excavators 3.3 按作业方式分类： classification by excavator operations 单斗与多斗挖掘机 single and multi-buckets excavators 3.4 按行走方式分类：Classification by running mode 履带式与轮胎式挖掘机（还有步履式、拖式、轨道式等） crawler and wheeled excavators (and walker, towed.Rail) null3.5 按动力方式分类：Dynamic classification by the way 内燃驱动（柴油机）与电力驱动挖掘机Diesel Driving (Diesel) and power-driven excavators 3.6 按使用条件方式分类：Under conditions of use methods to classification 专用型挖掘机（常说的矿山型）与通用型挖掘机（建筑型） Special type excavator (often say that the mine type) and Universal Excavator (BC-) 本文主要讲：单斗履带式液压挖掘机 This article stresses : Single Bucket tracked hydraulic excavatornull3.7 液压挖掘机的特点—与机械式挖掘机相比 Hydraulic excavator features - as compared with mechanical digger 比重量小（每1斗容量与整机机重之比）：重量可减轻一半the weight is small (each one fighting capacity and the engine aircraft weight ratio) : reduce the weight to half 挖掘力大(the strength of excavating)： 1.5:1 生产率高(Productivity)： 2.2倍以上 机动性和越野性能好 Good mobility and cross ability 工作平稳、安全可靠 Working stable, safe and reliable 结构紧凑、美观大方 Compact, attractive and unaffected null四 液压挖掘机总体结构 Overall structure of hydraulic excavatorsnull　　液压挖掘机的总体结构包括动力装置、工作装置、回转机构、操纵机构、传动系统、行走机构和辅助设备等，如图1-1所示。 The overall structure of the power hydraulic excavator, working equipment, rotary, manipulation, transmission,walking and auxiliary equipment, as shown in Figure 1-1. 常用的全回转式液压挖掘机的动力装置、传动系统的主要部分、回转机构、辅助设备和驾驶室等都安装在可回转的平台上，通常称为上部转台。 因此又可将单斗液压挖掘机概括成工作装置、上部转台和行走机构等三部分。Rotary hydraulic excavators used in the power plant, the main transmission system, Rotary,and auxiliary equipment installed in the control room, so rotatable platform, commonly known as the upper table.So it will be summarized into a single bucket of hydraulic excavator working device, the turntable and the upper part of the three agencies running. nullnull 图1-1 1配重 2发动机罩 3油水散热器总成 4发动机 5空滤 6蓄电池 7座椅 8液压油箱 9先导手柄 10燃油箱 11驾驶室 12回转支承 13工具箱 14中央回转接头15动臂油缸 16动臂 17斗杆油缸 18斗杆 19铲斗油缸 20摇杆 21连杆 22铲斗 23履带 24导向轮25张紧机构26支重轮 27托轮 28行走机构 29主泵 30回转机构 31回油滤清器 32主阀Map 1-1 1 weight 2 engine hood 3 oil-water radiators. Machines 4 Engine Air Filter 6 Battery 7 Seat 8 Hydraulic Tank 9 precursor of the handle 10 fuel tank 11 Cab 12 rotary supporting 13 toolbox 14 central swivel joints 15 Boom fuel tank 16 Boom 17 struggle rod fuel tank 18 bucket rods 19 bucket fuel tank 20 rocker 21 connecting rod 22 Bucket 23 tracked 24 guidance round 25 Tensioner 26 supporting wheels 27 child round of 28 running institutions 29 main pump 30 rotary bodies 31 back to the oil filter 32 main valvesnull动力系统 Dynamic Systemnull5.1 动力系统的基本知识 挖掘机动力系统的核心是柴油发动机。其功用是将供给的燃油燃烧并将热能转变为机械能，并通过液压传动系统使挖掘机行走和工作。 柴油机的动力输出就是液压泵的动力输入。 动力系统包括发动机及附件、燃油箱、水箱等。 Excavator basic knowledge of dynamical systems is the core of diesel engine power system.Its purpose is to supply thermal energy to mechanical energy will fuel combustion.Excavators and make walking and working through the hydraulic system.Engine power output is the input pump power.Annex including engines and power systems, fuel containers, water tanks, etc. null挖掘机动力装置由下列系统组成： 冷却系统、进气系统、 排气系统、燃油系统、 润滑系统、电器系统、 减震和传动系统 Excavator power plant consists of the following components : cooling system, intake system, exhaust system, fuel system, the lubrication system, the electrical system,damping and transmission system nullnull5.2 发动机编号 Engine No. 6BTA5.9-C# ------SY220C 6 — 6个汽缸； six cylinder B — 发动机系列 series of engine T — 涡轮增压器 Turbocharger A — 后级冷却（增压后中冷）After cooling (cold Turbocharged 5.9 — 51×51×3.14×120×6排量升51 × 51 × 314 × 120 × 6 displacement C — 改进次数 improved frequency # — 马力horsepowernull5.3 SY310挖掘机用柴油机介绍 SY310 excavator’s diesel 　　采用康明斯QSB5.9-240 型柴油机, 其可靠性、动力 性、燃油经济性和低排放性等指标在国际上都具有领先水 平。 Cummins Engine QSB5.9-240 use, reliability, power,Fuel economy and low emissions and so is a leading indicator of the level in the international arena. 　　柴油机主要技术参数：Diesel main technical parameters : 　　型式：六缸直列、水冷、直喷、涡轮增压、中冷、电控 Type : Inline six-cylinder, water-cooled, direct injection, turbocharger, cold, electric control 　　气缸直径(Cylinder diameter) (mm)×活塞行程(Piston stroke) (mm) ：102×120 　　额定功率 (Rated Power) (kW)/转速(rpm) ： 182/2000 　　最大扭矩Maximum torque (Nm)/转速 (rpm)： 990/1500 　　最低燃油消耗量Minimum fuel consumption (g/kWh)： ≤210null5.4 动力传输路线 表 关于同志近三年现实表现材料材料类招标技术评分表图表与交易pdf视力表打印pdf用图表说话 pdf power transmission line 行走动力传输路线：柴油机—联轴节—液压泵（机械能转化为液压能）—分配阀—中央回转接头—行走马达（液压能转化为机械能）—减速箱—驱动轮—轨链履带—实现行走 Power transmission line running :Diesel-coupling - hydraulic pump (mechanical energy into hydraulic energy) - valve-central swivel joints-running motors (hydraulic transformed into mechanical energy) - Gearbox-wheel-track crawler chain-operate to achievenull回转运动传输路线：柴油机—联轴节—液压泵（机械能转化为液压能）—分配阀—回转马达（液压能转化为机械能）—减速箱—回转支承—实现回转Rotary movement routes :Diesel-coupling - hydraulic pump (mechanical energy into hydraulic energy) - valve-rotary motors (hydraulic transformed into mechanical energy) - Reducer - Slew Ring-achieving Rotary. 动臂运动传输路线：柴油机—联轴节—液压泵（机械能转化为液压能）—分配阀—动臂油缸（液压能转化为机械能）—实现动臂运动 Boom campaign routes :Diesel-coupling - hydraulic pump (mechanical energy into hydraulic energy) - valve-Boom tank (hydraulic transformed into mechanical energy) - arm movements to achieve null斗杆运动传输路线：柴油机—联轴节—液压泵（机械能转化为液压能）—分配阀—斗杆油缸（液压能转化为机械能）—实现斗杆运动Stick campaign routes :Diesel-coupling - hydraulic pump (mechanical energy into hydraulic energy) - valve-Stick tank (hydraulic transformed into mechanical energy) - achieving struggle bar 铲斗运动传输路线：柴油机—联轴节—液压泵（机械能转化为液压能）—分配阀—铲斗油缸（液压能转化为机械能）—实现铲斗运动 Bucket campaign routes :Diesel-coupling - hydraulic pump (mechanical energy into hydraulic energy) - valve-tank Bucket (hydraulic transformed into mechanical energy) - bucket campaign to achieve null5.5 动力传输原理图Power transmission diagramnull 六 液压挖掘机的结构The structure of hydraulic excavators 液压挖掘机的结构件主要有底盘、回转支承、中央回转接头、转台、工作装置、配重、覆盖件、司机室、空调等。The structure is the major hydraulic excavator chassis, rotor support, the central swivel joints, turntable, the device weight, the panel,cab, air-conditioners, and so on. null6.1 底盘 底盘即行走装置, 包括履带架和行走机构, 主要由履带架、行走马达+减速机及其管路、驱动轮、导向轮、托链轮、支重轮、履带、张紧缓冲装置组成，见下图。其功能为支承挖掘机的重量，并把驱动轮传递的动力转变为牵引力，实现整机的行走。行走马达+减速机=行走机构 下车：底盘+回转支承+中央回转接头 Chassis is running devices, including planes and running track, tracked by planes, and operating a motor-reducer pipe.Sprockets orientation launches child sprocket, supporting wheels, tracks, tension device composed of the figure below.Its function is to support the weight of excavators, and wheel traction transfer of power to achieve a piece of the running. Motor running+ Reducer= running system. Alight: chassis + Slew Ring+Central swivel jointsnull底盘 chassisnullnull 支重轮 托链轮 导向轮 Supporting round. Asked sprocket. Steering round.null行走马达 驱动轮 张紧缓冲装置 Running motors, driving wheel. Tension devicenull履带架 Crawler Frame 　　履带架（即车架总成）为整体焊接件, 采用X 形结构，其主要优点是具有高的承载能力. 车架总成由左纵梁（即左履带架）、主车架（即中间架）、右纵梁（即右履带架）三部分焊接而成。 Tracked F (chassis assembly) welding pieces as a whole, the X-shaped structure.Its main advantages are high bearing capacity. Frame assembly from the left beam (left tracked F)Main Frame (intermediate F), right beam (right track F) three parts together through welding. nullnull中央回转接头Central swivel joints 　　中央回转接头是联接回转平台与底盘油路的液压元件，它保证回转平台旋转任意角度后，还能正常给行走马达供油。Central swivel joints are connected to the central Circuit hydraulic rotary platform and chassis components, which guarantees rotary platform arbitrary rotation angle,normal oil supply to the motor still running.Central swivel jointsnullnull6.2 平台 Platform 平台是上车部分主要承载结构件，分主平台和左右平台三部分。见下图。 Platform. The platform on the train carrying some of the major pieces of structure, principal platform and platform around three parts.See illustration below.nullnullnull6.3 工作装置Working Devices 　　工作装置是液压挖掘机的主要组成部分，目前SY系列挖掘机配置的是反铲工作装置，它主要用于挖掘停机面以下的土壤，但也可以挖掘最大切削高度以下的土壤， 　反铲工作装置由动臂、斗杆、铲斗、摇杆、连杆及包含动臂油缸、斗杆油缸、铲斗油缸在内的工作装置液压管路等主要部分组成。 Working devices is a key component of hydraulic excavators.SY series is currently configured backhoe excavator working device, it stands primarily for excavation below the soil surface.it is also the largest excavation of soil below the cutting height. Work by backhoe arm that will ban bucket, rocker,Boom linkage and includes tanks, fighting bar tank, the tank bucket of hydraulic pipe installation work and other major components. nullnull连杆机构Linkagenull提升负载提升负载提升负载动 臂Boomnull铲斗Bucket6.4 配重Weight6.4 配重Weightnull6.5 驾驶室与空调 驾驶室为流线形钢结构， 前窗挡风玻璃可滑动，天窗可开启，配置雨刮器、清洗器、收放机、空调、 座椅及操纵件、先导开关 4处出风口（除霜、吹脚、吹脸左、吹脸右）Cab and air-conditioning. Cab to streamline di e steel. Windscreen before sliding windows, skylights can be opened.Wiper allocation, 581, Radio and Tape Player, air conditioning. benches and manipulate documents, the pilot switches. four out of drafts (defrosting.Winds feet of the left face blowing and blowing face right)null空调系统Air-conditioning systemsnull空调系统的组成 1 压缩机总成 2 冷凝器总成 3 贮液干燥器总成 4 蒸发器总成 5 管路系统 6 电器系统 Air conditioning system :1 compressor assembly. 2 condenser assembly. Reliability three dry storage solution. 4 HVAC. 5 pipeline system. 6 electrical systemnull　　　七 挖掘机的液压系统 Excavators Hydraulic System null７.１液压系统概述 Hydraulic system. 液压元件组成：动力元件、控制元件、执行元件、辅件 动力元件：主泵 控制元件：主控制阀组 执行元件：回转马达、行走马达、动臂油缸、斗杆油缸、铲斗油缸。 辅件：油箱、油冷却器、蓄能器、电磁阀、比例阀、先导滤清器、中央回转接头、液压管接件等。 Hydraulic components : dynamic components and control components, the implementation of components, accessories dynamic components :main pump. Control components : the main control valve block. : rotary actuator motors running motors, Boom tank Stick tank bucket tank. Documentation : auxiliary fuel tanks, oil cooler, storage devices, solenoid valve, the proportional valve pilot filter, the central swivel joints, hydraulic tubing and parts. null液压系统分类：定量系统、变量系统、开式系统、闭式系 统、恒功率系统、双泵双回路系统、总功率调节、分功率调 节、中位、中位开心、中位闭心、节流调节系统、负流量控 制系统、正流量控制系统、负荷传感控制系统等。 Hydraulic System categories : quantitative system, variable system, open systems, closed system, constant power system, two double-loop pump systemtotal power conditioning, power adjustment minutes, the spaces, spaces unhappy that the median closed-center, cutting expenditure - conditioning systems, traffic control systems to negative.is flow control systems, load-sensing control system. null 　　液压系统是挖掘机的能源转换系统和各职能部件的控制 系统，由主油路系统和先导操纵系统组成。发动机的动力通 过液压系统转化为整机各职能部件运动的驱动力，并对各职 能部件的运动规律进行有效的控制。 The energy conversion system is a hydraulic excavator system components and functions of the control system.manipulated by the main oil system and the pilot system.Engine's hydraulic system through various functional components into a piece of the campaign driving forcealso all functional components of the movement for effective control.  null 主液压回路：用于主泵向各执行元件供油 先导液压回路：主要用于操作控制阀 挖掘机的液压系统通常由一台发动机、两个液压主泵、一个先 导泵、一个整体式控制阀组、四个液压油缸、一个回转马达和 二个行走马达组成。 Main : hydraulic circuit components for the implementation of oil supply to the main pump. Pilot hydraulic circuit :for the operation of the main control valve. usually by a system of hydraulic excavators engine, the two main hydraulic pump, a pilot pumpControl Valves a whole, four cylinder motor component of a rotary motor and walking two.null主泵通过联接盘直接与发动机输出轴相联。主泵通过控制阀 向动作执行元件供油；先导泵向三个先导阀供油。 先导阀：二个工作先导阀（实现挖掘机的各种动作）和一个 行走先导阀（实现挖掘机的行走与转向）。 Pump connected to the main disk directly associated with the engine output shaft.Through the implementation of actions to the main pump control valve components dispensing; Leading to the three pilot valve dispensing pump.Pilot valve :Working two pilot valve (excavators achieve various moves), and running a pilot valve (and to achieve the operating excavator). null挖掘机作业过程 挖掘机一个作业循环包括以下动作： １、挖掘——通过对铲斗、斗杆以及它们的复合动作，实现 铲斗的破土、装土。 ２、满斗回转——铲斗装满土后，动臂提升、同时进行平台 回转到卸土位置。 ３、卸土——平台回转到位后制动，由斗杆调节卸土半径， 铲斗翻转卸土。 ４、回位—— 铲斗卸土，转台反转，动臂、斗杆配合，回 到挖掘位置。 Excavator process. Excavator a circle of the following actions : 1, on tap -- Bucket.The composite rod movements and their struggle to achieve the momentum bucket, put the soil. 2, rotary -- mand bucket filled with soil, Boom upgrade, the platform back to the dumping place. 3, dumping -- rotating platform brake in place by the struggle radius rod regulate dumping, dumping bucket overturned. 4 spaces -- back bucket dumping, the table has turned around, arm, tie rod struggle to return excavation location.   null7.2 三一挖掘机液压系统介绍 Sany excavator hydraulic system. 　　　三一挖掘机采用国内外普遍使用的双泵双回路全功率 变量中位开芯系统，通过流量反馈信号控制油泵流量，使流 量按需分配到各职能部件，实现不同工况的最佳匹配。 　　　　　　　　　　　　　　　　　　　　　　　　　　　　　三一挖掘机液压系统主要有日本川崎公司提供。川崎公 司的液压系统采用负流量控制，动臂、斗杆的负载保持阀和 再生阀集成在主阀内，动臂、斗杆在主阀内实现合流。 Sany excavator uses abroad commonly , double-pump loop-power variable opened core systemfeedback signal to control the flow of oil flow, the flow of functions DAMA partsdifferent conditions to achieve the best match.March 1 excavator hydraulic system mainly for the Japanese company Kawasaki.Kawasaki companies in the hydraulic system used to control flow negative, Boom, fighting to maintain leverage load renewable valve integrated in the main valve and valve, Boom.the main valve lever in the struggle to achieve convergence. null挖掘机的电气控制系统 Excavators Electrical Control Systemnull8.1 电气控制系统概述 Electrical Control System 8.1.1 电气控制系统的功能electrical control system’s functions 挖掘机的电气控制系统主要实现以下功能： 电子监控、自动报警及故障自诊断 节能降耗、提高生产率 实现柴油机的控制，如电子调速器、电子油门控制装 置、自动怠速停机装置、自动升温控制装置 实现作业过程的自动化或半自动化 Excavators main electrical control system to achieve the following functions :electronic monitoring, automatic alarm and fault diagnosis. saving energy, increasing productivity. Diesel controlif the electronic speed governor, electronic throttle control device, the device automatically Idle stands,automatic temperature control device to achieve the automatic or semi-automatic operation processnull８.1.2 电气控制系统的使用条件Electrical Control System conditions of use. 电气控制系统应满足以下使用条件： 能在－４０℃ ～８０℃的环境温度下可靠、稳定的工作 抗老化、具有较长的使用寿命 密封性能好，能防止水分和污物的侵入 有较好的耐冲击和抗振性能 有较强的抗干扰能力，系统能在各种干扰下可靠地工作 Use electrical control system should meet the following conditions :in the ambient temperature range of -40 ° C to 80 ° C and reliable, stable jobs. aging.have a longer life expectancy. Seal,can prevent the intrusion of water and dirt. a better impact resistance and anti-vibration properties. a strong anti-jamming capability.the system can work reliably under all sorts of interference.null8.1.3　电气控制系统的组成 The electrical control system components. 挖掘机的电气控制系统基本上有以下二部分组成: 车辆电气设备 电源：包括蓄电池、发电机及其调节器。 起动机：用于起